In a CDMA mobile communication system, because of co-existing signals of multiple users in the same channel and complexity of the wireless environment, when a receiver receives an expected signal, it is unavoidable that the signal is affected by the fading, the Multiple Address Interference (MAI), and the Inter-symbol Interference (ISI). Utilizing interference information effectively, the multi-user detection algorithm eliminates or degrades the interference through de-spreading and multi-user joint channel equalization, so the system performance is improved greatly.
Before multi-user detection, the spreading codes and the corresponding channel responses of the users accessing in the current timeslot must be known by the receiver first. Usually, the channel responses can be obtained by channel estimation through training sequences. However, for the spreading codes, a terminal knows only about the spreading code used by itself and does not know about other users' spreading codes contained in received signals. Furthermore, on the one hand, when the traffic rate changes from large to small, there may be no transmission in some of the allocated channels; on the other hand, due to the complexity of the beam forming transmission and the wireless environment, some of the allocated channels may experience severe attenuation and fading. All these will result in the variation of the received signals of the mobile terminal.
At present, a higher layer signaling can be used to inform each mobile station of the necessary spreading codes for multi-user detection. However, its implementation is very difficult. The main reasons are: first, the real-time variation of received signals cannot be reflected; second, complexity of the signaling is too high; and third, load of the terminal is too heavy.
Therefore, in the wireless communication system, it is desirable that there is a method and apparatus for detecting the active code channels before multi-user detection so as to inform each mobile station ready to receive signals of the information of spreading codes needed in multi-user detection. Such information will not only reflect the real-time variation of received signals but also require lower signaling complexity and produce less heavy load for the terminals.